ERS2 radar data acquired before and after the Mw7.6, Manyi (Tibet)
earthquake of November 8, 1997, provide geodetic information about the
surface displacement produced by the earthquake in two ways. (1) The sub-pixel
geometric adjustment of the before and after images provides a two dimensional
offset field with a resolution of ~1m in range (radar line of sight) and
~20 cm in azimuth (satellite track direction). The offset map reveals a
smooth, N78E, ~170 km long surface rupture following the trace of a quaternary
fault visible on satellite images. The inferred sense of slip is left-lateral,
consistent with slip on the EW plane of the Harvard CMT solution. (2) Interferometric
processing of the data provides a range (radar line of sight) displacement
map with a precision of a few millimeters. The fault slip distribution
derived from the interferometric map is bimodal: a main event ruptured
the 130 km-long eastern section of the fault with a maximum slip of 7 m,
a sub-event ruptured the western 40 km-long section of the fault with up
to 2.6 m of slip. The observed asymmetry of the surface displacement field
between the two sides of the fault suggests that elastic properties of
the crust are dependent of its volume strain. Such a nonlinear elastic
behavior can be attributed to the presence of cracks in the crust. Simple
modeling suggests that a ratio of 2 to 4 between compressional and extensional
Young's moduli in the shallow part of the seismogenic crust can account
for the radar observations. Although extrapolating values of elastic parameters
derived from laboratory measurements to large volumes of crust must be
done with caution, a contrast of 2-4 between compressional and tensional
rigidity constants is commonly observed in laboratory test samples. The
occurrence of a large strike-slip event in this part of Tibet is the manifestation
of the left-lateral shear zone that extends from the Kunlun fault to the
western corner of the plateau, by-passing the Altyn Tagh fault.